Systems and methods for loop termination detection based on per-port calibration

ABSTRACT

Systems and methods for performing loop termination are described. One embodiment is a method that comprises receiving a per-port calibrated echo signal of a loop under test, receiving a region designation and a loop length for the loop under test, and determining whether the loop is terminated by a short or open termination based on phase of the per-port calibrated echo signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to, and the benefit of, U.S.Provisional Patent Application entitled, “REMEDY DSL LAYER-1 SELT-FDRANALYSIS ENGINE BASED ON PER-PORT CALIBRATION,” having Ser. No.60/905,474, filed on Mar. 7, 2007, which is incorporated by reference inits entirety.

TECHNICAL FIELD

The present disclosure generally relates to point-to-point wire linecommunications and more particularly to single end loop testing ofdigital subscriber line (DSL) communication systems.

BACKGROUND

Prior to deploying xDSL transmissions, a subscriber loop is generallyqualified or characterized by estimating the channel capacity, whichdepends on the transfer function of the network. The subscriber loop (orline) connects the customer premises (CP) to the central office (CO) andcan be affected by a wide range of impairments, including but notlimited to, load coils, bridge taps, mixed wire gauges, and bad splices.While the loop length and the wire gauge of the loop are generally notconsidered actual impairments, they also have a large impact on xDSLtransmissions. Other impairments include split pairs, untwisted dropcables, radio-frequency interference (RFI), and cross-talk. Moreover,several of the aforementioned impairments for xDSL transmissions are notpresent for POTS (plain old telephony service) because xDSL exploits amuch wider frequency band as compared to POTS. Consequently, theexisting POTS testing equipment is not capable of qualifying asubscriber loop for xDSL transmission. Due to these impairments, thexDSL network termination (NT) installed at the CP may in some cases noteven link up with the xDSL line termination (LT) in the DSL accessmultiplexer (DSLAM) at the CO. If the xDSL modems do link up with oneanother, there is no guarantee with respect to the quality-of-service(QoS) in terms of bit rate.

Qualifying a subscriber loop for xDSL requires estimating its channelcapacity, which depends on the attenuation of the line and also on thenoise power spectral density (PSD) at the CO for upstream transmissionand at the CP for downstream transmission, respectively. The estimate ofthe channel capacity of a particular loop/line will be most accurate ifthe transfer function of the line and the noise PSD at the CO and CP aremeasured directly.

Conventional methods for qualifying a subscriber loop include use ofxDSL test units available on the market that are capable of performingsuch measurements. In addition, these test units are often combined witha “golden” modem plug-in module that emulates a real xDSL modem of acertain type, such as ADSL, in order to estimate the real bit rateinstead of only the theoretical channel capacity. However, this approachrequires sending a technician to the CP, which is very expensive.Single-ended loop testing (SELT) can be used to extract informationabout the transmission environment (e.g., the loop) in a DSL system byperforming reflective measurements remotely at the CO/CP (or Modem)terminal, without the need to dispatch a technician. As an example, SELTmay comprise injecting signals into a loop under test at a centraloffice (CO) in order to determine the loop capability for supportingdifferent kinds of DSL services. As such, SELT often plays an importantrole in DSL provisioning and maintenance.

SUMMARY

Briefly described, one embodiment, among others, is a method thatcomprises receiving a per-port calibrated echo signal of a loop undertest, receiving a region designation and a loop length for the loopunder test, and determining whether the loop is terminated by a short oropen termination based on phase of the per-port calibrated echo signal.

Another embodiment is a method that comprises receiving an echo signalfor a loop under test, wherein the echo signal is a per-port calibratedecho response obtained using frequency domain reflectometry single-endedline testing (FDR-SELT). The method further comprises analyzing the echosignal and determining loop termination if the loop under test is notdetermined to be a long loop based on a predetermined threshold and ifno bridge tap is present on the loop.

Yet another embodiment is a system that comprises a first module coupledto the loop where the first module configured to generate a test signaland receive a reflected signal to determine an echo response of theloop. The system further comprises a second module configured to receivethe echo response measurement from the first module. The second moduleis configured to determine characteristics associated with the loopbased on the echo response and to output a termination type of the loopif the loop is not determined to be a long loop based on a predeterminedthreshold and if no bridge tap is present on the loop.

Other systems, methods, features, and advantages of the presentdisclosure will be or become apparent to one with skill in the art uponexamination of the following drawings and detailed description. It isintended that all such additional systems, methods, features, andadvantages be included within this description, be within the scope ofthe present disclosure, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the disclosure can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present disclosure. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 illustrates an xDSL system in which embodiments of SELT areapplied.

FIG. 2A depicts various components of the SELT module depicted in FIG.1.

FIG. 2B depicts the signal flow for the Layer 1 module shown in FIG. 2A.

FIG. 3 illustrates an embodiment of the SELT module shown in FIGS. 1 and2.

FIG. 4A-B are top level flowcharts for an embodiment of a process forperforming FDR-SELT utilizing the components depicted in FIGS. 1, 2A-B.

FIGS. 5 and 6 depict various plots of the S₁₁ signal as a function offrequency.

FIG. 7 is a flowchart for an embodiment for estimating the loop lengthbased on characteristics of the S₁₁, as depicted in FIG. 4A-B and asdescribed in relation to FIGS. 5-6.

FIGS. 8A-C depict another embodiment of a process for estimating theloop length as shown in FIGS. 4A-B.

FIG. 9 is a flowchart for one embodiment of a training phase used in theestimation of loop length as depicted in FIGS. 8A-B.

FIG. 10 is one embodiment of the noise removal module shown in FIGS.8A-B.

FIG. 11 illustrates the presence of a bridge tap between the CO and CP.

FIG. 12A illustrates the impact of a bridge tap on the absolute value ofthe calibrated echo (|S₁₁(f)|) when compared to straight loops.

FIG. 12B illustrates how the inflection points in |S₁₁(f)| translate tolocal maxima below 0 (or local minima above 0) in the derivative (firstdifference).

FIGS. 13A-C is an embodiment for detecting the presence of bridge tapsin a loop as shown in FIGS. 4A-B.

FIG. 14 shows the dependence of the amplitude of the reflected signal ondifferent termination types.

FIG. 15 shows various plots which illustrate wrapped and unwrapped phaseof S₁₁(f) corresponding to bin# 38 for North American gauges as afunction of loop-length.

FIG. 16A-E is an embodiment for determining the loop termination asshown in FIGS. 4A-B.

FIG. 17 is a plot of the absolute value for a calibrated S₁₁ responsefor a straight loop.

FIG. 18A-B is an embodiment for determining the loop gauge as shown inFIGS. 4A-B.

In order to facilitate a fuller understanding of the present disclosure,reference is now made to the accompanying drawings, in which likeelements are referenced with like numerals. These drawings should not beconstrued as limiting the present disclosure, but are intended to beexemplary only.

DETAILED DESCRIPTION

Having summarized various aspects of the present disclosure, referencewill now be made in detail to the description of the disclosure asillustrated in the drawings. While the disclosure will be described inconnection with these drawings, there is no intent to limit it to theembodiment or embodiments disclosed herein. On the contrary, the intentis to cover all alternatives, modifications and equivalents includedwithin the spirit and scope of the disclosure as defined by the appendedclaims.

In a wire-line broadband system, one of the primary objectives is toqualify a subscriber-loop for xDSL transmission by estimating itschannel capacity. This can be achieved if the loop characteristics suchas the topology, loop-length, gauge, and line-end status are known.These loop characteristics also serve as useful tools fortrouble-shooting and diagnostic purposes in the event that a provisionedservice experiences problems or fails. Accordingly, systems and methodsare described herein for analyzing the per-port calibrated echo (S₁₁(f))in the frequency domain to estimate the loop characteristics discussedabove. More specifically, the subscriber loop being characterized isidentified as a bridge-tapped (BT) loop, an inconsistent loop, or astraight loop. Furthermore, for loops classified as straight loops, thefollowing estimations are performed: loop length, loop gauge, andtermination type (i.e., open or short).

One technique used for gathering SELT measurements is frequency-domainreflectometry (FDR) where a frequency sweep is performed on the loopunder test in order to derive frequency-selective characteristics. Forexample, peaks in the measured receive signal correspond to frequenciesthat create standing waves. Measurement of the one-port scatteringparameter involves determining the echo response of the loop. Forpurposes of nomenclature used herein, the echo response may also bereferred to as S₁₁, S₁₁(f), or echo signal. Based on the echo response,the input impedance of the loop as a function of frequency can bedetermined from which loop characteristics can be estimated. Suchcharacteristics may include, for example, loop length and looptermination type.

When performing single-ended loop testing (SELT), various factors canaffect the accuracy of the measurements and ultimately affect anyestimations derived from the measurements. One factor is the presence ofa long loop where the measuring capability of the system is exceeded.Another factor relates to the topology of the loop itself (such as thosedue to bridge taps, for example). Accordingly, embodiments describedherein for performing SELT indicate when such factors exist, as they canresult in erroneous estimations. Systems and methods are described forcapturing and analyzing frequency domain reflectometry single-ended looptest (FDR-SELT) measurements to determine whether the responsecorresponds to a loop that has bridge taps. Exemplary embodimentsdescribed herein are based on the per-port calibrated FDR-SELT (S₁₁(f))echo in the frequency domain and in particular, on the actual analysisof the calibrated echo signal. Based on this analysis, valuable loopinformation can be derived from different characteristics of the S₁₁signal.

It should be noted that exemplary embodiments described herein may alsoinclude the ability to flag S₁₁ responses that are associated with verylong loops in addition to the ability to detect inconsistent loopestimates. For purposes of nomenclature used herein, the term “Layer 0”generally refers to the phase of FDR-SELT whereby a signal istransmitted into the loop under test and the scattering parameters arecaptured and calibrated. The term “Layer 1” generally refers to thepost-processing phase whereby the data captured in Layer 0 is used toderive meaningful information relating to the loop under test.

Reference is now made to FIG. 1, which illustrates an xDSL system inwhich embodiments of SELT are applied. In the non-limiting example shownin FIG. 1, N end users (or N sets of CPE 110 a, 110 b, 110 c) aredepicted where each user 110 a, 110 b, 110 c is referenced using anindex m. The end users 110 a, 110 b, 110 c are connected via a loop to acentral office (CO) 130, where a SELT module 132 for deriving loopcharacteristics may be implemented. The CO 130 may include an xDSLaccess multiplexer (DSLAM), xDSL line cards 140 a, 140 b, 140 c, andother equipment for interfacing with end users 110 a, 110 b, 110 c. Insome embodiments, the SELT module may be incorporated into xDSL linecards 140 a, 140 b, 140 c. In other embodiments, the xDSL line cards 140a, 140 b, 140 c may interface with the SELT module 132. It should benoted that while embodiments for SELT are described here in the contextof central offices, the principles of SELT contained in this disclosuremay also be incorporated into customer premises equipment as well.

The SELT module 132 first performs Layer 0 functions and injects a testsignal 120 a onto the loop under test. The reflected signal 120 b isthen measured to determine the echo response of the loop. The nature ofthe reflected signal 120 b will vary depending on the characteristics ofthe loop. FIG. 1 shows three non-limiting examples of various loopconfigurations. Loop 134 is a straight loop configuration, whereas loop135 and loop 136 depict a bridge tap loop and a long loop, respectively.Generally, the definition of a “long” loop may vary and is based on apredetermined threshold. As a non-limiting example, a loop that exceeds2 km may be categorized as a long loop.

FIG. 2A depicts various components for the SELT module depicted inFIG. 1. In accordance with exemplary embodiments, the SELT module 132may comprise a Layer 0 module 204 and a Layer 1 module 208. The Layer 0module 204 may further comprise a signal generator 220 and an analyzer222. The signal generator 220 transmits test signals on the loop undertest. The analyzer 222 monitors the reflected signal to derives-parameters associated with the network. The Layer 1 module 208receives information from the Layer 0 module 204, including a calibratedS₁₁ parameter. The Layer 1 module 208 may comprise a loop lengthestimator 224, a loop-type classifier 228, and a loop terminationestimator 230. For other embodiments, the Layer 1 module 208 may alsoinclude a loop gauge estimator 226 if a priori knowledge of the loopgauge is not available. Furthermore, the Layer 1 module 208 may alsoinclude a noise removal module 206.

FIG. 2B depicts the signal flow for the Layer 1 module shown in FIG. 2A.Prior to deriving characteristics associated with the loop, the Layer 1module 208 may receive such inputs as the region 232 associated with theloop. As non-limiting examples, the region may be designated as NorthAmerica or Japan. The calibrated S₁₁ parameter 235 is also forwarded tothe Layer 1 module 208. Finally, if a priori knowledge regarding theloop gauge is available, this information may also be forwarded to theLayer 1 module 208. Based on the inputs, the Layer 1 module 208 derivesinformation relating to the loop including, an estimation of the looplength 240, the termination type on the loop 241.

It should be emphasized that depending on whether the Layer 1 module 208determines that the loop under test is not a straight loop, the Layer 1module 208 may raise various flags 233 to denote that the estimationsmay be inaccurate. For example, in the event that the loop is determinedto be a long loop, the Layer 1 module 208 will flag that the measuringcapabilities of the overall system has been exceeded. As a result, anindicator or flag of some type may be raised. As another example, theLayer 1 module might raise a flag if the presence of a bridge tap isdetected on the loop under test or based on some anomaly observed in theS₁₁ signal, an inconsistency is detected in the loop under test. Thepurpose of these indicators or flags is to convey that the derived loopcharacteristics (e.g., loop length, termination type, loop gauge) mightnot be accurate.

Reference is now made to FIG. 3, which illustrates an embodiment of theSELT module shown in FIGS. 1 and 2. Generally speaking, the SELT module132 may be incorporated into the central office and can comprise any oneof a wide variety of computing devices. Irrespective of its specificarrangement, SELT module 132 can, for instance, comprise memory 312, aprocessing device 302, a number of input/output interfaces 304, anetwork interface 306, and mass storage 326, wherein each of thesedevices are connected across a data bus 310.

Processing device 302 can include any custom made or commerciallyavailable processor, a central processing unit (CPU) or an auxiliaryprocessor among several processors associated with the SELT module 132,a semiconductor based microprocessor (in the form of a microchip), amacroprocessor, one or more application specific integrated circuits(ASICs), a plurality of suitably configured digital logic gates, andother well known electrical configurations comprising discrete elementsboth individually and in various combinations to coordinate the overalloperation of the computing system.

The memory 312 can include any one of a combination of volatile memoryelements (e.g., random-access memory (RAM, such as DRAM, and SRAM,etc.)) and nonvolatile memory elements (e.g., ROM, hard drive, tape,CDROM, etc.). The memory 312 typically comprises a native operatingsystem 314, one or more native applications, emulation systems, oremulated applications for any of a variety of operating systems and/oremulated hardware platforms, emulated operating systems, etc. Forexample, the applications may include application specific software 316such as the Layer 0 module 204 and Layer 1 module 208 depicted in FIG.2A. One of ordinary skill in the art will appreciate that the memory 312can, and typically will, comprise other components which have beenomitted for purposes of brevity.

Input/output interfaces 304 provide any number of interfaces for theinput and output of data. With further reference to FIG. 3, networkinterface device 306 comprises various components used to transmitand/or receive data over a network environment. The SELT module 132 mayfurther comprise mass storage 326. For some embodiments, the massstorage 326 may include a database 328 to store and manage such data asmetadata.

FIGS. 4A-B provide a top level flowchart for an embodiment of a processfor performing FDR-SELT utilizing the components depicted in FIGS. 1,2A-B. In particular, FIGS. 4A-B is an embodiment of the overallanalysis/processing performed using the echo response obtained for aparticular loop. In accordance with exemplary embodiments, the stepsdiscussed below may be performed by the FDR-SELT module 132 shown inFIG. 2A. Based on the echo response, such metrics as loop length,termination type, and loop gauge can be derived. The presence of abridge tap can also be determined based on the echo response.

Step 402 begins by measuring or obtaining calibrated s-parameters. Instep 403, the relevant region (e.g., North America, Japan) is specifiedalong with the loop gauge, if available. Based on the S₁₁ parameteralong with the region and loop gauge, an estimation of the loop lengthis performed (step 404). Further, a determination is made on whether abridge tap exists on the loop (step 405). In particular, a determinationis made on whether a straight loop exists. Next, a determination is madeon whether the loop is a long loop (step 406). As emphasized earlier, ifthe loop exceeds a certain threshold, the loop is considered a long loopand the measuring capabilities system performing the test is exceeded.

If the loop does not exceed the predetermined threshold and a bridge tapis not present (decision block 407), then a loop gauge estimation isperformed if the loop gauge is unknown (step 408). An estimation of thetermination type (short vs. open termination) is also performed andforwarded as an output (steps 409, 410). Furthermore, the loop lengthestimate and loop gauge is output (step 410). Referring back to decisionblock 407, if the loop exceeds the predetermined threshold (i.e., theloop is a long loop) or if a bridge tap is present, then the methodproceeds to node B, depicted in FIG. 4B. If a long loop is present, aflag or some indication is made that a long loop exists (step 411).Likewise, if a bridge tap is present, then a flag is raised accordingly.Further, no attempt is made to determine the termination on the loop aseither the measuring capability of the system is exceeded at the end ofthe loop or a bridge tap is present (step 412). As such, an indicationis made that the exact loop length, the loop gauge, and the looptermination are unknown (step 412).

Various embodiments for estimating the loop length are now described.However, the basis for utilizing the calibrated S₁₁ parameter tocalculate a loop length is first discussed. By analyzing the one-portscattering parameter (S₁₁) of the loop under test as a function offrequency, certain behavior may be monitored which is dependent on theloop length. In particular, the period of ripples observed in theamplitude of the S₁₁ signal depends largely on the loop length and isgenerally independent of the particular loop gauge or termination type.As a result, the period of the ripples can be utilized to estimate theloop length.

Reference is made to FIGS. 5 and 6, which depict various plots of theS₁₁ signal as a function of frequency. As depicted in FIG. 5, the periodof the ripples (observed in the amplitude of the S₁₁ signal) depends onthe loop length and is largely independent of the loop gauge and looptermination. Further, as depicted in FIG. 6, a greater level ofattenuation in the S₁₁ signal is observed at higher frequencies forlonger loops. As a result, the S₁₁ signal experiences ripples withconsistent periods at lower frequencies for a relatively smallbandwidth. Thus, for exemplary embodiments, the observed bandwidth ofthe input signal is identified and restricted in order to observe a trueripple period. Generally, the process for estimating loop lengthinvolves a training phase, which is performed in order to obtainregion-specific coefficients. This information is later used when theloop under test is being analyzed. Reference is briefly made to FIG. 9,which is a flowchart for the training phase. During the training phase,the frequency of the ripple experienced by the S₁₁ signal as a functionof loop length is obtained for all loop gauges available (step 902).This serves as a reference or template for later estimating the looplength. Based on this information, the mean ripple frequency is computedas a function of loop length (step 904). Next, interpolation isperformed to obtain coefficients of a corresponding linear function(step 906). It should be noted that the training phase is only performedonce as the resulting coefficients may be stored for future reference.

Reference is now made to FIG. 7, which is a flowchart for an embodimentfor estimating the loop length based on characteristics of S₁₁, asdepicted in FIG. 4A-B and as further described in relation to FIGS. 5-6.Beginning in step 702, the calibrated S₁₁ parameter obtained throughSELT-FDR is received, and the relevant region or country associated withthe loop environment is specified. In the context of the embodimentsdescribed herein, the region will generally be specified as either“North America” or “Japan.” It should be emphasized, however, that theregions are not limited to these two designations and that the conceptsdescribed herein can be applied to other regions as well.

In accordance with some embodiments, the S₁₁ signal may be first passedthrough a noise removal module (step 704) in order to remove or reduceany noise that may affect later estimations made based on the S₁₁signal. In step 706, the ripple period associated with the S₁₁ signal ismeasured. In step 710, an estimated loop length is then calculated.

Reference is made to FIGS. 8A-B, which depicts another embodiment of aprocess for estimating the loop length. FIGS. 8A-B illustrates oneimplementation of the steps shown in FIG. 7. As described earlier, theloop length estimation is based on the periodicity of the ripplesobserved in the amplitude of the calibrated echo in the frequencydomain. This feature depends largely on the loop-length and is for mostpart insensitive to the gauge and termination of the loop. Beginning instep 802, the region (e.g., Japan vs. North America) is specified andthe calibrated S₁₁(f) parameter is provided. In steps 803 and 804, theenergy of the calibrated S₁₁(f) parameter in the relevant VDSL band iscalculated and used to normalize the calibrated S₁₁(f).

In step 805, the normalized S₁₁(f) is passed through the noise removalmodule. For some embodiments, the noise removal module may incorporatethree Hamming windows of lengths 51, 15 and 7 for filtering with athreshold of 2×10⁻⁴ for each filter. FIG. 10 provides an implementationof the noise removal process that may be utilized. As depicted in steps1002-1011, signal processing is generally performed on the normalizedS₁₁ response in order to eliminate noise, which can affect the accuracyof loop length estimates. While FIG. 10 shows signal processing usingHamming Windows, it should be noted that other means for removing noisefrom the normalized S₁₁ signal may be incorporated as well. After S₁₁ isfiltered, peak detection is performed via a primary peak detectionmodule in order to obtain a preliminary ripple-period estimate andseparation/gaps between consecutive peaks (step 806).

Referring back to FIGS. 8A-C, in decision block 807, if the preliminaryripple-period estimate is greater than a predetermined threshold (nodeB), the determination is made that a short loop is present (step 808).Next, depending on the specified region (e.g., North America vs. Japan),different sets of values are used to calculate the loop length (steps809, 810). As a non-limiting example, the predetermined threshold is setto 60, and the following equation is used to calculate the loop lengthusing the ripple-period (rpl_prd):

${Loop\_ Length} = {k\left( \frac{\left( {\frac{1}{rpl\_ prd} + a} \right)}{b} \right)}$where k=1, a=0.00002722, b=0.00004558, and the loop length is in metersif the region is “Japan” (step 809). If the region is specified as“North America”, then different values are used: k=3.281, a=0.00000641,b=0.00004708, and the loop length is in feet (step 810).

Referring back to decision block 807, if the preliminary ripple-periodestimate is less than or equal to a predetermined threshold (node C),the determination is made that an intermediate loop is present (step812). The normalized S₁₁ parameter prior to noise removal is used toobtain an updated estimate of the ripple period and gaps. The gaps arethen passed through a secondary peak detection module to get a finalripple period estimate (rpl_prd) (step 814). Next, depending on thespecified region, different sets of values are used to calculate theloop length (steps 816, 818). The loop length is calculated using thesame equation set forth for steps 809 and 810.

Exemplary embodiments for detecting bridge tap terminations are nowdescribed. With reference to FIG. 11, the presence of bridge tapsbetween the central office and customer premises can affect the qualityof service in a given subscriber loop. As such, methods for detectingbridge taps comprise detecting and discarding estimates where theresults of other estimations (e.g., loop-length estimation,gauge-detection, and termination detection) are likely erroneous ormisleading. It should be emphasized that this feature improves thereliability of the loop analysis.

The detection of a bridge tap loop relies on the presence of a number ofcharacteristics that are ideally not exhibited by straight loops. Forpurposes of nomenclature, these characteristics will be referred to as“differentiating features” or “differentiating characteristics” in thecontext of detecting bridge taps. With reference to FIG. 12A, thepresence of a bridge tap introduces abrupt changes in the amplitude andperiodicity of observed ripples in the absolute value of the calibratedecho (|S₁₁(f)|) when compared to straight loops. These abrupt changes in|S₁₁(f)| are the result of multiple reflections in the received echo.

The presence of a BT introduces inflection points in |S₁₁(f)|, which areideally not observed in straight loops responses. Accordingly, thepresence of such differentiating features are monitored in order todetermine whether a bridge tap exists. In particular, differentiatingfactors that relate to the maxima and minima of the absolute value ofthe calibrated echo and its derivative are examined. As illustrated inFIG. 12B, the inflection points in |S₁₁(f)| translate to local maximabelow 0 (or local minima above 0) in the derivative (first difference).

In accordance with exemplary embodiments, the presence of the followingdifferentiating factors/characteristics are monitored. The term“differentiating” is used as these features are used to differentiate astraight loop from a bridge tapped loop. The following differentiatingfactors may be monitored and used to determine the presence of a bridgetap:

-   -   1) a relevant minimum in the derivative of |S₁₁(f)| with value        greater than zero and both flanking minima less than zero;    -   2) a relevant maximum of the derivative of |S₁₁(f)| with a value        less than zero and both flanking maxima greater than zero;    -   3) an interval between two maxima in the derivative of |S₁₁(f)|        that is separated from the mean by more than the maximum of the        specified fraction of the standard deviation or a specified        number of samples;    -   4) an interval between two minima in the derivative of |S₁₁(f)|        that is away from the mean by more than the maximum of the        specified fraction of the standard deviation or the specified        number of samples;    -   5) a relevant maximum in |S₁₁(f)| higher than its previous        maxima by the specified tolerance; and    -   6) a relevant minimum in |S₁₁(f)| having value greater than the        specified threshold and with both flanking minima less than the        specified fraction of its value.        It should be noted that pre-processing of |S₁₁(f)| and its        derivative may be performed in order to filter out spurious        maxima/minima. The relevant bandwidth is then determined from        the derivative of |S₁₁(f)|. While it is generally important to        detect the presence of bridge taps, this requirement must be        balanced with the need to limit the probability of erroneously        flagging straight loops as bridge tapped loops. In some        instances, the latter requirement is more important (from the        customer's perspective) because for a large number of cases, the        estimated loop length remains fairly accurate for a bridge        tapped loop even if the presence of a bridge tap for that loop        is missed.

Based on the foregoing, exemplary embodiments for detecting the presenceof bridge taps are now described. Reference is made to FIGS. 13A-C,which depict an embodiment for performing the process discussed above.For preferred embodiments, the following inputs are provided: acalibrated S₁₁(f); a loop length estimate; and the region of deployment(step 1302). Next in step 1304, initialization is performed whereby thebins, window lengths, and thresholds is performed. In particular, thebins that define the edges of the DS1 frequency band and the 1 MHz binare initialized. The window is used for performing a moving average.Further, the appropriate region-specific thresholds and tuningparameters are obtained. (See Tables 1 and 2 below.) In step 1306, thelength of the smoothing filter is determined based on the loop length.Filtering is performed to remove any unwanted noise from S₁₁.

The determination of whether the loop length estimate is inconsistent ismade by correlating the loop-length estimate with an intrinsic parameterof the calibrated FDR echo that varies with loop-length but is yetindependent of the loop-length estimate, namely, the 1 MHz band energy.Regions of inconsistency are defined in the 1 MHz band energy vs. trueloop-length plane and any calibrated FDR echo for which the pair of 1MHz band energy and estimated loop length values falls within a regionof inconsistency is flagged accordingly. Exemplary embodiments forperforming these steps include the use of various thresholds andtolerances that are carefully tuned to increase the probability ofcorrect detection while at the same time limiting the probability oferroneously flagging a straight-loop as a BT-loop.

Based on the foregoing, if the loop is either too long or if the looplength estimate is inconsistent, then processing stops as an estimate ofthe termination type might yield an erroneous result. If the loop lengthis not too long and loop length estimate appears reliable, however,processing continues. A bridge tap is present if any of thedifferentiating factors listed earlier are exhibited. For someembodiments, a flag (e.g., BT_flag) may be set to indicate the presenceof a bridge tap.

In step 1308, elements of the S₁₁ response that fall outside the DS1 binare discarded or truncated, and the derivative for S₁₁ is computed. Insteps 1310 and 1312, the maxima and minima of S₁₁ and its derivative aredetermined and stored. The energy of S₁₁ in the band up to 1 Mhz iscomputed (step 1314). In step 1316, based on the loop length estimate,an indication is given (such as a flag) that a long loop is present.Processing then stops. If the loop length estimate itself is below apredetermined threshold, then the energy of S₁₁ is examined to determinewhether a long loop may nevertheless be present (in the event of anerroneous loop length estimate) (step 1318). In step 1320, the“relevant” bandwidth is determined based on the examining the identifiedmaximums. In step 1322, the relevant minima along with a running averageof the minima are calculated. At this point, an indication is given thata bridge tap is present if one of the relevant minimums is greater thana predetermined fraction of the running average.

An indication is also given if one of the relevant maximums is less thana predetermined fraction of negative[absolute value of the runningaverage for the maxima] and both flanking maxima are positive (step1326). In step 1328, various statistics, including the gaps betweenconsecutive values of relevant maxima, the average of these gaps, andthe standard deviation of the gaps are computed and used in determiningwhether a bridge tap is present (step 1330). Similar steps are performedfor relevant minimums where gaps between consecutive points areidentified, and statistical analysis is performed (steps 1332-1338).

Based on the various steps described above, a determination is made onwhether a bridge tap is present between the central office and thecustomer premises on the subscriber loop. As described above, theanalysis is based on various differentiating features found in thecalibrated S₁₁ response such as the identification of relevant minimumsand maximums and other peaks that flank these values. The analysisfurther involves statistical analysis with respect to these points.

Exemplary methods for determining the loop termination type are nowdescribed. Generally, determining the termination type of a loopinvolves determining whether the end of the loop comprises a short oropen termination, assuming that the loop is a straight loop (i.e., not abridge tapped loop) and that the loop is not a long loop. Generally aloop length greater than Threshold B (e.g., 6,600 ft) is considered tobe a long loop for North American loops. A loop greater than Threshold Y(e.g., 2 km) is considered to be a long loop for Japanese loops. Itshould be emphasized that while the embodiments below are described inthe context of the North American and Japanese regions, the conceptsdescribed herein can be applied to loops in other regions as well.

The loop termination type is also generally made based on a prioriknowledge of the loop gauge. The S₁₁ signal is utilized to determine theloop termination type because the phase of the reflected signal (i.e.,S₁₁ or echo response) depends on how the loop is terminated.Specifically, a straight loop terminated with a matched impedanceideally does not cause any echo, whereas a loop with either an open orshort termination causes a reflection in the transmitted signal. Thereflected signal suffers a phase inversion at the loop end if the loopend is short terminated. If the loop end is open terminated, thereflected signal does not suffer any phase inversion at the loop end.Accordingly, FIG. 14 illustrates the interleaved pattern in theamplitude of the S11 signal, caused due to the phase variation in theecho-response, corresponding to the two termination types. This is incontrast to the case of a matched termination, which ideally does notgenerate any echo.

Determining the termination type of a particular loop first involves atraining phase whereby a set of frequency bins depicting consistent,calibrated S₁₁ phase responses as a function of loop-length isidentified. FIG. 15 shows various plots which illustrate wrapped andunwrapped phase responses corresponding to bin# 38 for North Americangauges versus Loop-length. Next, the various thresholds (which will beregion-specific) corresponding to the chosen bins are obtained. (Thethresholds, however, may or ma not be gauge-specific.) Forimplementations involving North American and Japanese loops, it shouldbe noted that a priori knowledge of the loop gauge is not mandatory forloop lengths less than or equal to Threshold X (e.g., 850 m) for Japanand Threshold A (e.g., 2,800 ft) for North America). That is, thethresholds are not gauge specific. However, knowledge of the loop gaugeis necessary for loop lengths which exceed these values as thethresholds are gauge specific. Finally, independent decisions made withrespect to one or more of the frequency bins are examined to determinethe loop termination type.

FIG. 16A-E is one embodiment for determining the loop termination inaccordance with the concepts described above. As depicted in FIG. 16A,the following inputs are received in step 1602: a calibrated S₁₁(f); aloop length estimate; the loop gauge; and the region of deployment.Information derived from a training phase helps identify frequency binsdepicting consistent, calibrated S₁₁ phase responses as a function ofloop-length. It should be noted that the training phase is not part ofthe actual loop termination process as this is only performed once inorder to derive information used for determining the loop termination.Depending on the specified region (e.g., North America vs. Japan)(decision block 1606), the loop length estimate is compared to variousthresholds to determine the loop termination type. For the embodimentdepicted in FIGS. 16A-C, FIG. 16B illustrates the process involved ifthe specified region is North America, and FIG. 16E illustrates theprocess involved if the specified region is Japan.

With reference to FIG. 16B, a determination is first made if the looplength exceeds Threshold A (e.g., 2,800 ft) in decision block 1608. Forembodiments involving North American loops, if the loop is not longerthan Threshold A, the loop termination type can be made independently ofthe loop gauge (step 1609). If the loop exceeds Threshold A (e.g., 2,800ft), then a check is made on whether the loop exceeds Threshold B (e.g.,6,600 ft) in decision block 1610. If the loop is longer than ThresholdB, then processing stops as depicted in step 1612 since the measuringcapabilities of the SELT-FDR analysis engine are exceeded. In suchcases, the termination type is returned as “unknown.” In step 1614, theparticular loop gauge is required as the loop is greater than ThresholdA. If the loop gauge is not known, then processing again terminates andthe termination type is unknown. If the loop gauge is known, thenprocessing continues (step 1616).

With reference to FIG. 16C, a set of frequency bins that exhibit aconsistent phase response is acquired as a function of loop length andloop gauge (step 1618). A series of phase responses are considered to beconsistent if they provide clear separations or gaps between open andshort terminations. As discussed earlier, the phase of a reflectedsignal for an open termination will be out of phase with respect to thephase of a reflected signal for a short termination for the same looplength. Next in step 1620, thresholds for the frequency bins areobtained for various loop lengths. Based on the information discussedabove, a correlation can be made such that the loop termination type canbe identified (steps 1622, 1624). With reference to FIGS. 16D and 16A,if the loop length estimate is less than Threshold A, the loop gauge isnot required. Steps similar to those discussed for FIG. 16C are followed(steps 1626-1632).

FIG. 16E depicts an implementation for Japanese loops. First, acomparison is made on whether the loop exceeds Threshold X (decisionblock 1634). In accordance with some embodiments, Threshold X may be setto a value of 850 m. If the loop length does not exceed Threshold X,then the loop termination type can be determined even if the loop gaugeis unknown (step 1644). If the loop is a long loop (i.e., exceedsThreshold Y (e.g., 2 km for Japanese loops), then the termination typeis returned as unknown (decision block 1636, step 1638). If the looplength falls between Threshold X and Threshold Y, then the loop gauge isrequired, otherwise processing terminates and the termination type isunknown (steps 1640, 1642).

Exemplary embodiments for determining the loop gauge are now described.Generally, information on the particular loop gauge can be valuableinformation as the loop gauge can be used to determine the data-ratethat can be supported on a line. The loop gauge can also be used toguide decision-making in cases involving service disruption. Asnon-limiting examples, the loop gauge may be determined to be 24 AWG, 26AWG for North American loops and 0.4 paper, 0.4 poly, 0.65 poly forJapanese loops.

Through the S₁₁ response, the loop gauge may be determined based on thefact that for a given loop length, the gauge of a particular loopaffects the amplitude of the calibrated echo signal. The absolute valuefor a calibrated S₁₁ response for a straight loop at 200 m is plotted asa function of frequency in FIG. 17. Thus, by analyzing certaincharacteristics of the FDR-SELT echo, thresholds can be incorporated todifferentiate between different gauges for various loop lengths. SuchFDR-SELT characteristics are dependent on the loop gauge and include thefollowing: 1) the energy up to 1 MHz; 2) the envelope of maxima andminima; and 3) the span of the ripples of S₁₁. It should be noted thatsince any one feature by itself does not provide sufficientdifferentiation between different gauges across all frequencies and looplengths, the loop gauge is determined based on a merging criterion ofone or more individual decisions based on these characteristics. Thishelps to increase robustness via diversity if more than one feature isused to determine the gauge.

In accordance with exemplary embodiments for performing loop gaugeestimation and with reference to FIGS. 18A-B, the following inputs areutilized: 1) the calibrated echo response S₁₁(f); 2) the loop lengthestimate; and 3) the region of deployment (e.g., North America vs.Japan) (step 1802). Using the loop length estimate, an appropriate setof discriminating features or metrics are chosen. Variouscharacteristics in the calibrated echo are monitored and compared withtabulated thresholds in order to obtain feature-wise decisions. Theindividual feature-wise decisions are then merged to obtain an overallgauge decision.

As depicted in FIG. 18A, the following are initialized: 1) the bins thatdefine the edges of the DS1 band (DS1_start=33, DS1_end=512); 2) the bincorresponding to 1 MHz (bin_(—)1 MHz=232); and 3) the variable thatdetermines the tolerance used in determining the thresholds for thegauge-detection routine (tol=0.3). Next, the region-specificcheck-frequencies (e.g., check_freqs=250, 500, 1000 kHz for “Japan”;check_freqs=250, 300, 1500 kHz for “North America”) are initialized(steps 1803, 1804). In step 1806, the energy for the per-port calibratedS₁₁ parameter is computed for the DS1 band as well as the energy in thebands up to 1 MHz. The variable S11_DS1 is set to the absolute value ofthe S₁₁ parameter in step 1808. This value is then normalized andfiltered prior to processing (steps 1810, 1812), resulting in S11_NR.The maxima and minima of this value (S11_NR) are then determined byperforming envelope detection and stored (step 1814).

In step 1816, the intercepts of the maximum and minimum envelope at theregion-specific check-frequencies are computed and stored. The span ofthe envelope at each check-frequency is computed (step 1818). Based onthe information derived in the preceding steps, Table 1 (below) is usedto determine the metrics for deriving a loop gauge estimate (step 1820).

TABLE 1 Region and Supported Range of Useful Gauges Metric Lengths North1 The minimum-envelope intercept of [200 ft-400 ft] America:abs(S11_cal) at 1500 kHz ‘24 AWG’, 2 The energy of S11_cal in the fre-[400 ft-2400 ft] ‘26 AWG’, quency band up to 1 MHz and (i.e., E_1M)‘unknown’. 3 The maximum-envelope intercept of [600 ft-3200 ft]abs(S11_cal) at 300 kHz 4 The maximum-envelope intercept of [1800ft-4000 ft] abs(S11_cal) at 250 kHz 5 The envelope span of abs(S11_cal)[3400 ft-6600 ft] at 300 kHz Japan: 1 The maximum-envelope intercept of[100 m-700 m] ‘0.4 Poly’, abs(S11_cal) at 1000 kHz ‘0.4 Paper’, 2 Theenergy of S11_cal in the fre- [150 m-500 m] ‘0.65 Poly’, quency band upto 1 MHz and (i.e., E_1M) ‘unknown’ 3 The maximum-envelope intercept of[200 m-1100 m] abs(S11_cal) at 500 kHz 4 The maximum-envelope interceptof [650 m-1400 m] abs(S11_cal) at 250 kHz 5 The envelope span ofabs(S11_cal) [1300 m-2000 m] at 250 kHzTable 1 is a list of the various region-specific metrics and the rangeof loop-lengths over which these metrics are useful to discriminatebetween gauges. As an example, for a loop length estimate between 400and 2,400 ft, metric 2 (energy of S11_cal in the frequency band up to 1MHz) would be used to determine the particular loop gauge.

For each of the valid metrics, the exact decision-thresholds for theestimated loop length are computed via linear interpolation. Further,the observed value of the valid metric is compared with thecorresponding exact decision-thresholds to obtain an estimate of thegauge and stored in the variable gauge{j}, where j is the index of thevalid metric. Finally, the various gauge estimates stored in gauge{ }are merged to obtain a final gauge estimate (final_gauge) based on therules given in Table 2 below. As discussed earlier, the loop gauge isdetermined based on a merging criterion of one or more individualdecisions based on these characteristics (step 1824, 1826).

TABLE 2 Number of valid metrics Merging rule 1 (trivial Final gauge issimply the same as the sole intermediate gauge decision merge) 2 a) Ifboth intermediate gauge-decisions are the same: The final gauge equalseither of the intermediate gauge decisions b) If exactly oneintermediate decision is ‘unknown’: The final gauge equals the otherintermediate decision (The ‘unknown’ decision is discarded). c) If thetwo intermediate decisions are different and neither is ‘unknown’, thefinal gauge is ‘unknown’. 3 a) The final gauge decision equals the gaugedecision determined by the majority of the three valid metrics. b) Ifthe three metrics give three different intermediate gauge-decisions, thefinal gauge is ‘unknown’ (This is the 1-1-1 case). 4 a) If three or morevalid metrics give the same intermediate gauge decision, the final gaugedecision equals the decision of the clear majority (This is the 3-1 or4-0 case). b) If two of the valid metrics concur on an intermediategauge decision, but the other two valid metrics do not concur, then thefinal gauge decision equals the most common decision, i.e., thatrepresented by the two valid metrics that concur. (This is the 2-1-1case). c) If two of the valid metrics concur on one intermediate gaugedecision, while the other two concur on a different intermediate gaugedecision, then the final gauge decision is ‘unknown’. (This is the 2-2case). d) If all four valid metrics give different intermediate gaugedecisions, then the final gauge decision is ‘unknown’. (This is the1-1-1-1 case).Table 2 above provides a list of rules for deriving the final gaugeestimate based on merging the gauge estimates of the valid metrics(called intermediate gauge decisions).

It should be noted that the invention is not limited to the embodimentsdescribed above. Further, it should be noted that while the processesherein are described for the North American and Japanese regions, manyvariations and modifications may be made to the above-describedembodiments without departing from the principles of the presentdisclosure such that other regions can be supported. Accordingly, itshould be emphasized that the above-described embodiments are merelyexamples of possible implementations. All such modifications andvariations are intended to be included herein within the scope of thisdisclosure and protected by the following claims.

The invention claimed is:
 1. A method comprising: receiving a per-portcalibrated echo signal of a loop under test; receiving a regiondesignation and a loop length for the loop under test; and determiningwhether the loop is terminated by a short or open termination if theloop is not determined to be a long loop based on a predeterminedthreshold and if no bridge tap is present on the loop, whereindetermining whether the loop is terminated by a short or opentermination is based on phase characteristics of the per-port calibratedecho signal.
 2. The method of claim 1, wherein the predeterminedthreshold is one of 6,600 ft and 2km.
 3. The method of claim 1, furthercomprising receiving a loop gauge of the loop under test if the loop isnot a long loop but the loop length is greater than a secondpredetermined threshold.
 4. The method of claim 3, wherein the secondpredetermined threshold is one of 2,800 ft and 850m.
 5. The method ofclaim 1, wherein the region designation is one of: North America andJapan.
 6. The method of claim 1, wherein determining whether the loop isterminated by a short or open termination comprises: identifying a setof frequency bins as a function of loop length in the echo signal thatexhibit a consistent phase response such that the consistent phaseresponse provides clear separation between open and short terminations;obtaining one or more phase thresholds for the frequency bins as afunction of loop length; and based on the relationship of the phase ofan echo response with respect to the phase thresholds, determining thetermination type of the loop.
 7. The method of claim 6, wherein the setof frequency bins and the one or more phase thresholds aregauge-specific.
 8. A method comprising: receiving an echo signal for aloop under test, wherein the echo signal is a per-port calibrated echoresponse obtained using frequency domain reflectometry single-ended linetesting (FDR-SELT); and analyzing the echo signal and determining looptermination if the loop under test is not determined to be a long loopbased on a predetermined threshold and if no bridge tap is present onthe loop.
 9. The method of claim 8, further comprising: specifying aregion; and receiving a loop length estimate, wherein the looptermination is further determined based on the region and the looplength estimate.
 10. The method of claim 9, wherein the region comprisesone of: North America and Japan.
 11. The method of claim 9, whereindetermining loop termination comprises determining whether the loop isterminated by an open termination or a short termination by observingthe phase of the echo signal and correlating it with an expected phaseof the echo signal based on measurements taken at the same loop lengthfor different terminations.
 12. The method of claim 9, furthercomprising: determining whether the loop exceeds a first threshold; inresponse to the loop not exceeding the first threshold, determining theloop termination independent of loop gauge; in response to the loopexceeding the first threshold, receiving a loop gauge and determiningwhether the loop is a long loop based on a second threshold anddetermining the loop termination based on the loop gauge if the loop isnot a long loop.
 13. The method of claim 12, wherein determining looptermination further comprises: identifying a set of frequency bins as afunction of loop length and loop gauge in the echo signal that exhibit aconsistent phase response such that the consistent phase responseprovides clear separation between open and short terminations; obtainingphase thresholds for different ranges of loop length; and mergingindependent decisions obtained for the set of frequency bins todetermine the loop termination type.
 14. The method of claim 13, whereinthe phase thresholds are gauge-specific.
 15. A system comprising: afirst module coupled to a loop, the first module configured to generatea test signal and receive a reflected signal to determine an echoresponse of the loop; and a second module configured to receive the echoresponse measurement from the first module, the second module configuredto determine characteristics associated with the loop based on the echoresponse, the second module configured to output a termination type ofthe loop if the loop is not determined to be a long loop based on apredetermined threshold and if no bridge tap is present on the loop. 16.The system of claim 15, wherein the first module comprises: a signalgenerator to send the test signal onto the loop; an analyzer configuredto measure scattering parameters (s-parameters) based on the reflectedsignal and determine the echo response of the loop.
 17. The system ofclaim 15, wherein the second module is further configured to receive aspecified region associated with the loop.
 18. The system of claim 17,wherein the region designation is one of: North America and Japan. 19.The system of claim 15, wherein the second module comprises: logic forreceiving a loop length estimate and a loop gauge; logic for identifyinga set of frequency bins as a function of loop length and loop gauge inthe echo signal that exhibit a consistent phase response such that theconsistent phase response provides clear separation between open andshort terminations; logic for obtaining phase thresholds for differentranges of loop length; and logic for merging independent decisionsobtained for the set of frequency bins to determine the loop terminationtype.